Start with a tsp a day, then work up from there.
I usually bf, then offer soilds about half an hour later. You don't want to wait too long or your baby will end up replacing a bf meal with solids, and you don't want that. You still want breast milk to be the main source of nutrition. Let baby fill up on bm, then give a tsp of solids.
I recommend giving the solids at breakfast, so you can monitor your child during the day for any reactions to the food.
Mashed bananas or avocados are a great first food. Just mash with a fork and dilute with breast milk to get the consistency you need.